The present invention relates to the field of powering systems, and in particular to an arrangement enabling the use of a driver having a limited voltage range in a system exhibiting a higher voltage.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LEDs are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display. In a large matrix display typically the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current.
In order supply a white backlight for the matrix display one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the matrix display, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused by a diffuser so as to avoid hot spots. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a correct white point.
Each of the colored LED strings is typically intensity controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance. AM is typically used to set the white point produced by disparate colored LED strings by setting the constant current flow through the LED string to a value achieved as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point among the disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight by adjusting the average current. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor. The color sensor is arranged to receive the white light, and thus a color control feedback loop may be maintained. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point.
In an embodiment in which single color LEDs, such as white LEDs are used, a similar mechanism is supplied, however only the overall luminance need be controlled responsive to a photo-detector. It is to be noted that as the single color LEDs age, their luminance is reduced as a function of current.
As described above, for many lighting applications including those for a matrix display, LEDs are often supplied in one or more LED strings, driven by a suitable PWM driver. The number of LEDs that are connected in series to form an individual string establishes the maximum voltage drop of the LED string, and thus is an important component in specifying the voltage output of the driving power source. In many applications large LED strings are preferred, as this simplifies the number of LED string drivers required. Unfortunately, LED string drivers, being constituted of an integrated circuit, are typically limited in the amount of voltage that may be applied to any of the pins of the LED string driver. This voltage limit is reflected in a limitation of the voltage output of the driving power source, since when the driver is in an off condition, a minimal voltage drop is experienced across the individual LEDs of the LED string and the voltage output of the driving power source is experienced at the LED string driver input.
One prior art solution to this problem entails inserting a high value resistor between the end of the LED string and ground, thereby pulling a dark current through the LEDs so as to develop a minimal voltage drop across the LED string sufficient to prevent an unacceptable voltage rise. Such a solution however leads to early burn out of the LED string, since the LEDs are not put into a fully off condition during the off portion of the PWM cycle.
What is needed, and not supplied by the prior art, is a method of extending the voltage range of an LED string driver so that the LED string driver may be utilized with a driving power source whose output exceeds the voltage rating of he LED string driver.